Various materials for making contact lenses are known in the art. The earliest contact lenses were made of glass material. However, the glass caused severe eye irritation such that the lenses could not be worn for extended periods of time. To overcome this disadvantage, later-developed contact lenses were made of polymethyl methacrylate (PMMA). However, early PMMA lenses did not allow passage of oxygen to the cornea, which caused eye discomfort and edema.
A contact lens material must not only have an acceptable level of oxygen permeability but, in addition, must have a high refractive index to provide visual acuity at both near and far distances, an adequate surface wettability, a good dimensional stability, a resistance to surface deposits, and must be durable to withstand handling.
In the latter part of the 20th century, other polymeric materials were developed for use in contact lenses. For example, in 1999, silicone hydrogel materials were developed. However, although silicone hydrogels provided increased oxygen permeability, the silicone made the lens surface hydrophobic and less wettable, resulting in discomfort and dryness during wear. This less-than-desirable dryness has resulted in limited acceptance of such lenses in the marketplace.
Polymeric materials have also been used to make bifocal contact lenses. For example, a single polymer or copolymer such as a methacrylate may be used. However, in such lenses, the near vision portion must be relatively thick, which can cause user discomfort due to increased pressure on the cornea.
It is also known to use two polymeric materials to make bifocal contact lenses. For example, the contact lens may comprise polymethyl methacrylate for the distance vision portion of the lens (refractive index: 1.49) and polystyrene (refractive index: 1.59) for the near vision portion of the lens. However, contact lenses formed in this manner have low oxygen permeability which can also result in discomfort and edema.
Other ways have been suggested for making two-piece bifocal lenses. For example, U.S. Pat. No. 5,170,192, (Pettigrew et. al.), entitled “Oxygen Permeable Bifocal Contact Lenses and their Manufacture” discloses a two-piece oxygen permeable bifocal contact lens by forming a low index cross-linked oxygen permeable copolymer preform with a recess, and casting a high index material therein. However, such two-piece lenses still suffer from inadequate oxygen permeability through the high index segment. These lenses also often experience warping, ruptures and other dimensional instabilities.
Accordingly, it would be advantageous to provide a monofocal and bifocal ophthalmic lens material which has good optical properties, including optical clarity and high refractive index and, in addition, acceptable oxygen permeability without the disadvantages of the conventional materials noted above.